The present invention relates to a superconducting magnet assembly of a structure in which leakage of a magnetic field can be suppressed successfully. More particularly, the invention is concerned with an active shielding type superconducting magnet (SCM) assembly.
As an example of the apparatus to which the present invention can be applied, mention is made of a magnetic resonance imaging apparatus (also simply referred to as MRI). In such an MRI apparatus, it is desirable to provide magnetic shielding such that the leakage of the magnetic field of the SCM assembly incorporated therein is confined as much as possible within the space in which the SCM is installed. For example, a pacemaker implanted in a patient with heart disease may be adversely influenced due to the leakage of the magnetic field. Most of the SCM assemblies used conventionally are of a self-magnetic shielding type. As is disclosed in U.S. Pat. No. 4,587,504 to Brown et al (issued May 6, 1985 and corresponding to JP-A-62-123756) and JP-A-62-5161 laid open on Jan. 12, 1987, the active shielding type SCM assembly is certainly advantageous over the self-shielding system realized with the aid of sheet iron in the respect that the former can be implemented having a lighter weight.
By way of example, in the case of an exemplary application in which the intensity of a static magnetic field (H.sub.o) generated within an internal magnetic space of a main coil of the SCM assembly is 1.5 T (Tesla), the iron material required for the self-shielding system amounts to about 30 to 60 tons, thus involving considerable difficulty in the installation of the self-shielding system. In contrast, the active shielding system requires no iron material and may provide a SCM assembly on a scale of 10 tons or less. Accordingly, it is expected in the future that the active shielding system will be increasingly employed as the magnetic shielding of the SCM in view of the increasing demand for a higher intensity static field (H.sub.o). The conventional, active shielding systems however are not considered to be satisfactory with regard to the items mentioned below.
Reference is made to FIG. 1 of the accompanying drawings which shows schematically a cross-section of an active shielding type SCM assembly. As will be seen in this figure, a superconducting (SC) cancel coil 2 is coaxially installed on a superconducting (SC) main coil for the purpose of generating a magnetic field having a direction opposite to that of the abovementioned static field (H.sub.o). Hence, the effective static field (H.sub.o) is given by EQU H.sub.o =A-a (1)
(wherein A represents the intensity of magnetic field generated by the SC main coil 1, and a represents the intensity of the opposite magnetic field generated by the SC cancel coil 2).
Thus, it will be understood that for generating a given intensity of the static magnetic field (H.sub.o), the SC main coil 1 is necessarily of a considerably large structure when compared with the system in which no active shielding system is adopted (i.e. a=0). The SC cancel coil 2 is also of relatively massive structure. For these reasons, the following problems are;
(i) The diameter of the SCM assembly is increased as a whole, involving a corresponding increase in the height of the SCM assembly which depends on the diameter (e.g. height of 30 to 40 cm in the case of a SCM assembly of 0.5 T). As a result, the SCM assembly can no larger be accommodated within an ordinary chamber or room (having a ceiling height of 2.4 to 2.7 m). In other words, a limitation is imposed on the environmental conditions for the installation of the SCM assembly More specifically, a room having a higher ceiling is required for installing the SCM system.
(ii) When compared with the, SCM assembly without the provision of the active shielding system, at least double the amount of expensive superconducting wire material is required, involving high costs in manufacturing the SCM assembly, which presents of course an obstacle to extensive utilization of the SCM system.
In FIG. 1, a reference numeral 3 denotes an internal magnetic space formed in the main coil 1 in which the static magnetic field (H.sub.o) is generated, 4 denotes a hermetically sealed cylindrical casing which may be made of, for example, stainless steel, 5 denotes a vessel for liquid helium, 6 denotes a vessel for liquid nitrogen, 7 denotes a bore defining the abovementioned internal magnetic space, A' and A" represent the leakage magnetic fields of the superconducting (SC) main coil, a' and a" represent the leakage magnetic fields of the superconducting (SC) cancel coil 2, LHe represents liquid helium, and LN.sub.2 represents liquid nitrogen.
As will be seen from the foregoing description, no consideration is given to the environmental condition for installation of the active shielding type SCM system which is too tall and is expensive.
JP-A-62-169311 laid open on July 25, 1987 discloses a structure in which a cage-type porous superconducting (SC) shielding is provided so as to enclose a SC main coil for suppressing generation of the leakage magnetic field.